1. Field of the Invention
The present invention relates to a valve utilizing shape memory alloys and an anti-lock brake system with the valve, and more particularly to a valve utilizing electrically-controlled shape memory alloys and an anti-lock brake system provided with the valve.
2. Description of the Prior Art
A shape memory alloy denotes an alloy that preserves a shape deformed by an external force below a critical temperature, whereas a shape memory effect of the alloy is activated for recovering a memorized original shape by a shape recovering force after being heated up to the critical temperature. Shape memory alloys such as a titanium-nickel alloy and an aluminum alloy are manufactured to have a predetermined shape at a high temperature. Such a shape memory alloy is utilized for valves of various types, and cooperates mechanically or electrically with the valve for moving elements of the valve to a predetermined direction to open/close ports of the valve.
There are methods for applying heat upon the shape memory alloys: one is to permit fluid to flow around the shape memory alloys to vary the temperature of the fluid, and another is to permit current to flow in the shape memory alloys to generate heat by an electrical resistance of the shape memory alloys.
FIG. 1 illustrates one example of a valve 10 for subjecting the shape memory alloys to heating by varying a temperature of fluid flowing around the shape memory alloys.
In FIG. 1, a spring 12 is in the shape of a coil spring which is manufactured by using a shape memory alloy. The fluid flows around spring 12. Once a temperature of the fluid is raised to reach a critical temperature of the shape memory alloy that initiates the shape memory effect, spring 12 is compressed by the shape recovery force to open valve 10. Meanwhile, when the temperature of the fluid is lowered, spring 12 is relaxed by a bias spring 14 to close valve 10.
However, the conventional valve 10 utilizing the above-described shape memory alloy spring 12 has drawbacks of difficulty in accurately controlling the opening range of valve 10 as well as a slow response speed of spring 12 with respect to the fluid temperature. Additionally, it involves a fastidious manufacturing process since the shape memory alloys must be shaped as the coil.
A valve for improving the above-stated problems is disclosed in U.S. Pat. No. 5,211,371 (issued to Coffee). Shape memory alloys utilized in the valve of Coffee are in the shape of a wire which is electrically-controlled by an electric circuit. The electric circuit is a closed circuit comprising a plurality of transistors and a plurality of capacitors, so that the shape memory alloys are actuated in conformity with a cycle by using operations of charging/discharging the capacitors and switching the transistors.
In the valve of Coffee, however, the valve should be continuously in the open state for maintaining a prescribed pressure. Furthermore, the shape memory alloys should be continuously supplied with current to maintain the open state of the valve. In this case, not only the power is significantly dissipated due to the continuous supply of the current, but also the control of the opening/closing operation of the valve by using the current supply is difficult while the shape memory characteristic is likely to be lost.
U.S. Pat. No. 5,092,901 (issued to Hunter et al.) describes shape memory alloy fibers with very short total contraction and relaxation time suitable for being employed as an electro-magnetic actuator. However, Hunter et al. do not specially disclose a valve utilizing the shape memory alloy fibers.
On the other hand, as is widely known, an anti-lock brake system (ABS) indicates a break system for preventing slipping along a road surface of wheels which are locked by the operation of the brake, and for improving a steering property. Especially, the anti-lock brake system affords an effective braking force and a steering capability in case of a sudden stop, braking on a curvy road, a wet road in the rain and an icy ground, and the like.
In the conventional anti-lock system as described above, pressure-regulating valves of respective wheels are formed of two valves of a diaphragm pattern, i.e., a pressure-holding valve and an exit valve. The pressure-holding valve and exit valve are controlled by two solenoid valves. In a normal braking operation, operation fluid flows to a brake cylinder via the pressure-regulating valves to realize the braking operation. Meantime, if one of the wheels is too abruptly locked, an electronic control unit (ECU) of the anti-lock brake system operates two solenoid valves to control the pressure within the brake cylinder and repeats locking and unlocking of the wheel at a very high speed, so that the slipping of the wheels is prevented while enhancing the steering capability.
However, the solenoid valves utilized for the anti-lock brake system have a complicated structure to require a demanding job in designing and manufacturing process with the consequence of high cost. In connection with the aforesaid valve that utilizes the shape recovery force of the shape memory alloy attributed to the temperature variation of the fluid, the shape memory alloy having a slow response speed with respect to the fluid temperature cannot be employed for the anti-lock brake system which requires a high-speed operation. Moreover, the above valve of Coffee has a difficulty in controlling the opening/closing operation of the valve which is unsuitable for the anti-lock brake system. Further, even though the shape memory alloys of Hunter et al. having short contraction and relaxation time are employed in the anti-lock brake system, the continuous current supply as described above necessarily results in the risks of significant power dissipation and possible loss of shape memory characteristic.